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14 July 2018

2016 AP Physics 1 problem 5 waves on a vertical string - video from Walter Keeley

In my PWISTA* AP Summer Institute, participant Walter Keeley tried to set up the waves-on-a-vertical-string problem from the 2016 P1 exam.  (Click the link, and go to problem 5.)  The problem asks students to explain, in a paragraph, how a picture of standing waves on a vertical string provides experimental evidence that the wave speed depends directly on the tension in the string.

*Putnam-Westchester Industry & Science Teacher Alliance


The correct approach:  The picture shows a longer wavelength in the standing wave near the top, where the tension in the significantly-massive string is greater.  Since the frequency of vibration is provided by the wave generator and is the same throughout the string, by v=λf the larger wavelength means larger wave speed.


Walter initially had no more success setting this up than I had over the years.  The strings I've used have simply not been heavy enough.  Walter tried a seriously massive rope, but it wasn't flexible enough to show a clear pattern.

It was Lorren, one of our participants, who suggested using ball-chain to provide both mass and flexibility.  It was Tom who pointed out that you'd have more luck seeing the wave if the generator didn't merely vibrate left-and-right, but in a small circle.

So, when Walter had some downtime at robotics camp, he obtained some chain (from True Value Hardware at 40 cents per foot).  Walter had to build his own geared motor to vibrate the chain:
Photo credit Walter Keeley

Walter used his phone to create a video of the vibrating chain; the app Hudl Technique is what he (and my students, too) use for playback, because it does slow motion and precise timing.

And oh my goodness, look at Walter's video: 

Look how you can see -- and measure, if you want -- the wavelength change.  Walter even arranged - presumably by trial and error with the frequency selection - to have the same three-and-a-half antinodes that are shown in the original AP problem.

Do you have a beautiful experimental setup of a released AP Physics problem?  Send pictures, and I'll be happy to post it on the blog with credit to you.  If you've never set up released AP problems in your lab, well, that's an awesome April open-ended lab assignment.


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